Adding `fine_scale=TRUE` feature

This release includes a substantial change to functionality in VAST, by now including variation in density occurring at finer spatial scales that the spatial resolution of knots. It does this by using bilinear interpolation (a "predictive process modelling" framework") to interpolate Gaussian Markov random fields from the location of knots to the location of samples and extrapolation-grid cells, and also by using a new interface to density covariates, replacing X_xtp (the value of covariates at each knot) with two inputs X_itp (the value of covariates at observations) and X_gtp (the value of covariates at extrapolation-grid cells).